Category Archives: Disease – Field and Forage

Disease - Field and Forage, Misc

Huitlacoche

July 24, 2015 hudelson

UW Plant Disease Facts

Authors: Joe Spraker*, UW-Madison Plant Pathology

Last Revised: 03/01/2024

D-number: D0065

Huitlacoche can be an excellent source of carbohydrates, proteins, fats, vitamins and minerals. (photo courtesy of CIMMYT)

What is huitlacoche?

Huitlacoche is the Native American name for the mushroom-like masses found on the ears of corn and corn relatives (collectively called teosinte). Huitlacoche can be a major problem for sweet corn growers where it is referred to as common corn smut (see UW Plant Disease Facts D0031, Common Corn Smut), causing yield and quality losses. However, huitlacoche is also considered a culinary delicacy in many cultures. In Mexico, huitlacoche is considered a gift from the gods, and is commercially available in many markets. In the United States, huitlacoche is less common commercially, but can be found in the ethnic sections of some grocery stores.

What does huitlacoche look like?

Huitlacoche is a tumor-like growth (called a gall) that forms from corn kernels. Galls can swell to ½ to 11 inches in diameter. Huitlacoche initially has a streaked, silvery appearance, but eventually develops a scorched, blackened look. Huitlacoche galls remain smooth until they rupture releasing a black, inky material that eventually becomes sooty, coarse and dry.

Where does huitlacoche come from?

Huitlacoche is actually a corn disease, caused by the fungus Ustilago maydis. The black, inky material released by huitlacoche is a mass of spores produced by the causal fungus. These spores can be easily moved by wind and can survive for many years in soil. Normally U. maydis infects unpollinated corn silks, but the fungus also can infect through wounds caused by heavy wind, heavy rain, or hail.

What can I do with huitlacoche if I find it?

Huitlacoche is edible, has a smoky, earthy taste, and is an excellent source of carbohydrates, proteins, fats, vitamins and minerals. To enjoy the flavor of huitlacoche to its fullest, harvest galls before they begin to dry, but after they have turned dark grey. DO NOT use galls that are powdery inside when split open. If you prefer not to harvest your own huitlacoche, you can purchase canned hutlacoche at your local Mexican market. Although use of hutlacoche as a food originated in Mesoamerica (i.e., parts of Mexico and Central America), like many other gourmet mushrooms, chefs have begun to incorporate huitlacoche into non-traditional dishes including macaroni and cheese, and even ice cream. See http://www.sweetcorn.illinois.edu/Common-smut/Recipes.htm for sample huitlacoche recipes.

How do I encourage more huitlacoche to form?

huitlacoche can be harvested from naturally infected ears of corn, you can increase your yield by artificial inoculation. Prior to pollination, soak corn silks with huitlacoche spores mixed with water. After inoculation, water and fertilize corn plants for optimal growth. If your inoculation is successful, galls should start to form within two weeks. Galls are usually best if harvested 16 to18 days after inoculation.

For more information on huitlacoche:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*Completed as partial fulfillment of the requiremen ts for Plant Pathology 558 at the University of Wis consin Madison.

An EEO/Affirmative Action employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. This document can be provided in an alternative format by calling Brian Hudelson at (608) 262-2863 (711 for Wisconsin Relay).

References to pesticide products in this publication are for your convenience and are not an endorsement or criticism of one product over similar products. You are responsible for using pesticides according to the manufacturer’s current label directions. Follow directions exactly to protect the environment and people from pesticide exposure. Failure to do so violates the law.

Thanks to Erica Arcibal, Cary Miao, Aracely Portillo, Mike Rankin, Robyn Roberts, Mary Stys and Trisha Wagner for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Broad Leafed Woody Ornamental, Disease - Field and Forage, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Specialty Crop, Disease - Vegetable

Herbicide Damage

July 24, 2015 hudelson

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 03/01/2024

D-number: D0060

What is herbicide damage?

Herbicide damage is any adverse, undesired effect on a plant that is caused by exposure of that plant to a pesticide designed for weed control (i.e., an herbicide). Any plant can be subject to this problem.

Squash leaf distorted due to exposure to a common lawn herbicide.

What does herbicide damage look like?

Symptoms of herbicide damage vary depending upon the plant affected and the herbicide used. Common symptoms include stems that are flattened, or that twist or corkscrew. Leaves may have abnormal shapes, sizes or textures. In addition, leaves or leaf veins may yellow or redden. In severe cases, plants may brown and die. Some plants, such as tomatoes and grapes, are particularly susceptible to herbicide damage and can be used as indicators of unwanted herbicide exposure.

How does herbicide damage occur?

Herbicide damage results when an herbicide is misapplied. Herbicides for control of broadleaf weeds are occasionally applied with fertilizers as part of a lawn care program. If these products are applied too close to ornamentals or vegetables, or are applied when there is too much wind, then the herbicide can drift (move) from the area of application into a non-treated area. Often, drifting herbicides are difficult to detect by eye because they are extremely fine mists. They can better be detected by smell. Some herbicides readily produce vapors that can begin to drift several hours after application.

How do I save a plant that has been damaged by herbicides?

There is nothing you can do after plants have been exposed. However, most plants accidentally exposed to broadleaf herbicides applied with lawn fertilizers do not receive a high enough dose to kill them. Young growth exposed to the herbicide will be distorted and discolored, but subsequent growth will be normal.

How do I avoid problems with herbicide damage in the future?

When using a lawn herbicide, follow the application directions exactly. DO NOT apply the product too close to, or in a manner that will cause exposure to, non-target ornamentals or vegetables. To avoid drift, apply the herbicide when there is as little wind as possible (< 5 mph). Apply the herbicide at low pressure to minimize production of fine mists. Finally, use amine forms rather than ester forms of herbicides as amine forms are less likely to produce vapors.

For more information on herbicide damage:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Sharon Morrisey, John Stier, Ann Wied and Chris Williamson for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Field and Forage

Fusarium Head Blight (Scab)

July 23, 2015 hudelson

UW Plant Disease Facts

Authors: Damon Smith, Trenton Stanger*, and Craig Grau, UW-Madison Plant Pathology

Last Revised: 03/01/2024

D-number: D0054

Bleached spikelets on a wheat grain head due to Fusarium head blight.

What is Fusarium head blight?

Fusarium head blight (FHB) or scab is a fungal disease that affects wheat, barley, oats, and many grasses. FHB is important, not only because it reduces yield, but because it reduces the quality and feeding value of grain. In addition, the FHB fungus may produce mycotoxins, including deoxynivalenol (also known as DON or vomitoxin) that, when ingested, can adversely affect livestock and human health. Maximum allowable levels of DON in feed for various animals (as set by the U.S. Food and Drug Administration) are ≤ 10 parts per million (ppm) for beef and feedlot cattle, ≤ 10 ppm for poultry, and ≤ 5 ppm for swine and all other animals.

What does Fusarium head blight look like?

Diseased spikelets on an infected grain head die and bleach prematurely. Healthy spikelets on the same head retain their normal green color. Over time, premature bleaching of spikelets may progress throughout the entire grain head. If infections occur on the stem immediately below the head, the entire head may die. As symptoms progress, developing grains are colonized by the FHB fungus causing them to shrink and wrinkle. Often, infected kernels have a rough, sunken appearance, and range in color from pink or soft gray, to light brown.

Where does Fusarium head blight come from?

FHB is caused by the fungus Fusarium graminearum, which is not only a pathogen of wheat, but also of corn. The fungus can overwinter in infested stubble and straw of cereals and weed grasses, and on stalks and rotted ears of corn. The severity of FHB varies greatly from year to year. Infection is favored by extended periods of high moisture or high (>90%) relative humidity, and moderately warm temperatures (59 to 86°F), particularly when these conditions occur just before or during wheat flowering (Feekes 10.5). If favorable weather conditions persist, infections can continue to occur through the early dough stage (Feekes 11.2). Extended periods of infection can be especially problematic in wheat stands where plants have varying levels of maturity.

Fusarium head blight can cause grain heads to become completely bleached.

UW Plant Disease Facts

How can I save a small grain crop with Fusarium head blight?

Fungicides are available for FHB control. Triazole fungicides [Fungicide Resistance Action Committee (FRAC) class 3] are recommended. In particular, fungicides containing prothioconazole and tebuconazole, or a mix of these two compounds, have provided the best control of FHB in university research trials. Avoid using strobilurin fungicides (FRAC class 11) as research indicates that use of these products can result in an increase in DON levels in harvested grain. A web-based FHB risk assessment tool (http://www.wheatscab.psu.edu) is available to help make decisions about fungicide applications. The tool also provides real-time, local commentary by extension personnel about the status of diseases in wheat. When using fungicides for FHB control, be sure to read and follow all label instructions to ensure that you use the product in the safest and most effective manner possible.

How can I avoid problems with Fusarium head blight in the future?

DO NOT plant small grains into small grain or corn residue. Also, avoid planting grain crops near areas where there are large amounts of small grain or corn residue on the soil surface. When possible, plant small grains following a legume crop (e.g., soybeans), and maintain a rotation with two to three years between small grain crops. Deep plow all infested plant debris, where feasible. DO NOT apply manure containing infested straw or corn stalks onto fields planted to small grains. Certain grain varieties have moderate levels of resistance to FHB. Consider using these varieties as a means to reduce disease severity and increase grain quality. Finally, plant several varieties of a small grain that vary in flowering date. This will decrease the risk that an entire crop will be vulnerable to FHB when weather conditions favor the disease.

For more information on Fusarium head blight:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*The original version of this fact sheet was completed as partial fulfillment of the requirements for Plant Pathology 559 – Diseases of Economic Plants at the University of Wisconsin Madison.

Thanks to Diana Alfuth, Bryan Jensen and Jim Stute for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional agriculture-related questions at https://extension.wisc.edu/agriculture/ask-an-agriculture-question/.

Disease - Field and Forage

Ergot

July 23, 2015 hudelson

UW Plant Disease Facts

Authors: Sarah Rosenthal*, UW-Plant Breeding and Genetics Dan Undersander, UW-Madison Agronomy Damon Smith, Craig Grau and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0050

Production of honeydew (red arrow) and sclerotia (white arrows) are typical of ergot.

What is ergot?

Ergot is a fungal disease of worldwide distribution that is common in the northern two-thirds of North America. Ergot affects wild and cultivated grasses, as well as small grain crops such as wheat, oats, barley and especially rye. The ergot pathogen produces alkaloid toxins, many of which reduce blood flow in animals (e.g., cattle, sheep, swine, horses and even humans) that eat ergot-contaminated grain. Ergot poisoning is cumulative. Symptoms can develop rapidly if animals eat large quantities of ergot or more slowly if they eat small quantities of ergot on a regular basis. In many animals (e.g., cattle), the first symptom of ergot poisoning is lameness that occurs two to four weeks after ergot is first eaten. Gangrene in extremities (e.g., hooves and ears) follows. Dairy cows that eat ergot-contaminated grain typically have a marked reduction in milk yield. Other symptoms of ergot poisoning can include convulsions, hallucinations and death. Symptom development is often more severe in very hot or very cold weather. Interestingly, some ergot toxins, when purified and used at low dosages, have pharmaceutical applications (e.g., inducing labor and treating migraine headaches).

What does ergot look like?

Signs of ergot first appear as droplets of a sticky exudate (called honeydew) on immature grain heads. Honeydew contains asexual spores of the ergot fungus. Over 40 species of insects are attracted to honeydew and can carry spores from infected to healthy plants. After approximately two weeks, infected grains are replaced by dark (often purplish), compact fungal structures called sclerotia. Sclerotia range in size from ¹/₁₆ to ¾ inches in length, and often look like seeds, rodent droppings, or insect parts.

Where does ergot come from?

Ergot is caused by several species of the fungus Claviceps, most commonly Claviceps purpurea. Sclerotia of these fungi survive in soil and harvested grain. Sclerotia require a one to two month period of cold temperatures (32 to 50°F), after which they germinate to form small, mushroom-like structures that produce sexual spores (different from those produced in honeydew). Germination occurs most commonly in cool (57 to 84°F), damp weather and is inhibited at higher temperatures. Sexual spores are blown to developing grain heads where infection occurs. Humid weather (> 90% relative humidity) contributes to honeydew production. Ergot is also often more severe if frosts occur at the time of spore production.

Ergot sclerotia germinate to form mushroom-like, spore-producing structures.

How can I save a small grain crop with ergot?

Fungicide treatments are not recommended to control ergot. When ergot is a problem, you can attempt to remove sclerotia from grain using commercial seed-cleaning equipment. However, if sclerotia are broken or are the same size as the grain itself, this may be difficult and costly, and may still result in grain that has a contamination level that is above marketable thresholds (tolerance for ergot sclerotia in harvested grain can be as low as 0.05% by weight). Ultimately destroying the contaminated grain may be the best course of action. Be sure to also destroy the hay from the affected field. DO NOT use the hay as feed or for animal bedding.

How can I avoid problems with ergot in the future?

Maintain a rotation with at least one year between small grain crops. Use crops that are not susceptible to ergot (e.g., soybeans, alfalfa, corn) in years when small grains are not grown. Plant grain seed that is free of ergot sclerotia. Ergot-resistant varieties are not available, but avoid longer-flowering varieties as they tend to be more susceptible to infection. Keep weed grasses under control. Also, mow areas adjacent to small grain fields to prevent grasses from flowering and developing ergot. In fields where ergot becomes a problem, consider clean, deep plowing that will bury ergot sclerotia to at least three to four inches, thus preventing sclerotia from germinating. Mow, remove and destroy ergot-infected grasses from pastures and hayfields. DO NOT allow animals to graze in these areas and DO NOT use the harvested material for feed or as bedding material.

For more information on ergot:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*The original version of this fact sheet was completed as partial fulfillment of the requirements for Plant Pathology 559 – Diseases of Economic Plants at the University of Wisconsin Madison.

Thanks to Diana Alfuth, Bryan Jensen and Kevan Klingberg for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional agriculture-related questions at https://extension.wisc.edu/agriculture/ask-an-agriculture-question/.

Disease - Broad Leafed Woody Ornamental, Disease - Field and Forage, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Specialty Crop, Disease - Vegetable

Downy Mildew

July 21, 2015 hudelson

UW Plant Disease Facts

Authors: Jim Olis* and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0044

What is downy mildew?

The downy mildews are a group of diseases that cause destruction of the leaves, stems, flowers and fruits of many plant species worldwide. In Wisconsin, downy mildews have traditionally been problems on grapes, cucumbers (see UW Bulletin A3978, Cucurbit Downy Mildew: Identification and Management, available at https://learningstore.extension.wisc.edu/), roses and Viburnum. Downy mildews on basil (see UW Plant Disease Facts D15, Basil Downy Mildew) and impatiens (see UW Plant Disease Facts D66, Impatiens Downy Mildew) have more recently become problematic.

What does downy mildew look like?

Downy mildew symptoms begin as small, green or yellow, translucent spots that can eventually spread to an entire leaf, stem, flower or fruit. Infected plant parts may eventually brown or bronze. The causal organism appears on infected stems, flowers and fruits, and on undersurfaces of infected leaves, as a downy, white, gray or purple fuzz.

Where does downy mildew come from?

Downy mildew is caused by several closely related fungus-like water molds (e.g., Peronospora spp., Plasmopara spp. and Pseudoperonospora spp.) that survive in plant debris or on infected plants. Downy mildew organisms are fairly host specific. The downy mildew organism that infects one type of plant (e.g., rose) is not the same downy mildew organism that infects another (e.g., grape). However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favorable for development of downy mildews on a wide range of plants.

How do I save a plant with downy mildew?

Downy mildews can be controlled using fungicides, if treatments are applied early enough in disease development. Fungicides containing chlorothalonil and copper (particularly Bordeaux mix, a combination of copper sulfate and lime) are labeled for downy mildew control in home gardens. Timing and number of applications will vary depending on exactly the type of downy mildew you are attempting to control. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with downy mildew in the future?

Consider buying downy mildew resistant varieties, when available. In new plantings, space plants far apart. In established plantings, prune or thin plants to increase airflow. Proper spacing and thinning will reduce humidity and promote rapid drying of foliage, which is less favorable for downy mildew development. Avoid overhead watering and apply water directly to the soil at the base of your plants. At the end of the growing season, remove and destroy infected plant debris as this can serve as a source of spores for the next growing season.

For more information on downy mildew:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*Completed as partial fulfillment of the requirements for a BS in Plant Pathology at the University of Wisconsin Madison.

Thanks to Lisa Johnson, Ann Joy and Kristin Kleeberger for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Broad Leafed Woody Ornamental, Disease - Field and Forage, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Misc, Turf

Dodder

July 21, 2015 hudelson

UW Plant Disease Facts

Authors: Jennifer Clifford, Amilcar Sanchez, Trenton Stanger, and Brian Hudelson UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0129

Spaghetti-like dodder plants parasitizing carrots.

What is dodder?

Dodder is the name of several species of parasitic plants that are widely distributed in North America and Europe. Plants parasitized by dodder include alfalfa, carrots, onions, potatoes, cranberries, a variety of herbaceous and woody ornamentals, and many weed species. Parasitized plants become weakened, have reduced yields (in the case of agronomic crops), and can potentially die.

What does dodder look like?

Dodders lack roots and leaves, and also lack chlorophyll, the green pigment found in most plants. Dodders have slender, yellow-orange stems that cover infected plants in a spreading, tangled, spaghetti-like mass. From May through July, dodders produce white, pink, or yellowish flowers.

Where does dodder come from?

Dodders produce large numbers of seeds that germinate in the spring to produce shoots that attach to suitable host plants. Dodders penetrate host tissue, and absorb nutrients via specialized structures called haustoria. Once established on a host, the bottom of a dodder plant dies (thereby severing its connection with the soil), and the dodder plant becomes dependent on the host plant for water and nutrients.

How do I save plants parasitized by dodder?

On woody ornamentals, simply prune out dodder-parasitized branches. When small patches of dodder occur among herbaceous plants, apply contact herbicides such as 2,4-D early in the season, preferably before dodder seedlings have parasitized host plants. Keep in mind that use of contact herbicides will likely also kill host plants. Alternatively, cut or burn dodder and parasitized plants to keep dodder from spreading, and to prevent seed production. For widespread dodder infestations, a combination of frequent tilling, burning and herbicide applications may be needed to achieve control. Be sure to read and follow all label instructions of the herbicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with dodder in the future?

Dodder’s wide host range and ability to survive as dormant seeds in soil make eradication difficult. Preventing introduction of dodder is the best method of control. Use dodder-free seed, and be sure to clean equipment thoroughly after working in a dodder-infested area. Try to restrict animal movement between infested and non-infested areas as well. Depending upon the specific crop or location, use of pre-emergent herbicides containing DCPA, dichlobenil, propyzamide, or trifluralin may be possible to prevent germination of dodder seeds. Destroy actively growing dodder and any parasitized plants before the dodder produces seeds. In agricultural settings where dodder has been a problem, rotate away from susceptible crops and grow non-host crops (e.g., corn, soybeans, or small grain cereals). In conjunction with rotation, adequate control of weed hosts is critical to achieve control.

For more information on dodder:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Lis Friemoth, Matt Lippert and Dan O’Neil for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Field and Forage, Disease - Vegetable

Corky Ringspot

July 14, 2015 hudelson

UW Plant Disease Facts

Authors: Brian Hudelson and Amy Charkowski, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0034

What is corky ringspot?

Corky ringspot (also known as spraing) is a potentially serious viral disease of potato that has recently been detected in Wisconsin. The disease can cause severe losses due to the fact that it reduces potato tuber quality, making tubers unsuitable for use in potato chip production and undesirable to consumers as table stock. Variants of this disease [usually referred to as tobacco rattle (see UW Plant Disease Facts D0116, Tobacco Rattle)] affect a variety of other plants including vegetable crops (e.g., beans, beets, peppers, and spinach), many herbaceous ornamentals (e.g., astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip, and vinca) and many weed species (e.g., chickweed, cocklebur, henbit, nightshade, pigweed, purslane, prickly lettuce, shepherd’s-purse and sowthistle).

Internal necrosis of tubers, often in fleck or arc patterns is typical of corky ringspot.

What does corky ringspot look like?

Symptoms of corky ringspot vary depending on the variety/cultivar of potato affected, and depending on environmental conditions. Foliar symptoms are rare, but on occasion can include reduced leaf size, puckering and mottling (i.e., blotchy light and dark coloring). More commonly, corky ringspot manifests itself underground as corky arcs, rings or flecks that form on or within tubers. Thinner-skinned and lighter-colored potato varieties are more likely to exhibit obvious ring symptoms on the surfaces of tubers. Symptoms similar to those caused by corky ringspot can be caused by other potato viruses such as alfalfa mosaic virus, potato mop-top virus, and certain strains of potato virus Y.

Where does corky ringspot come from?

Corky ringspot is caused by the Tobacco rattle virus (TRV) which is spread primarily by stubby-root nematodes, a group of microscopic, worm-like organisms in the genera Trichodorus and Paratrichdorus. These nematodes feed on the roots of infected plants (vegetables, ornamentals or weeds), acquiring TRV, then move to non-infected plants where their subsequent feeding spreads the virus. TRV also can be spread mechanically when knives or other tools that are used to cut tubers for seed pieces, or that are used to divide ornamental plants, become contaminated. In addition, on ornamentals, TRV can be spread by pruning and grafting, and via movement of seed from infected plants.

How do I save potatoes with corky ringspot?

Once potatoes have become infected with TRV, they remain infected indefinitely. Infected plants cannot be treated in any way to eliminate the virus and should be removed and disposed of by burning (where allowed by local ordinance), burying or composting. Before destroying symptomatic plants, you may want to have them tested to verify the presence of TRV. Note that ELISA (a technique commonly used to test for other potato viruses) is not a reliable test for TRV; polymerase chain reaction (PCR) should be used to test for this virus. Once TRV is introduced into a field, it is likely to remain there indefinitely. Stubby-root nematodes can carry the virus for extended periods and weed species can serve as reservoirs of the virus indefinitely.

Tubers with corky ringspot may, but do not always have target-like ring patterns on their surfaces.

How do I avoid problems with corky ringspot in the future?

The best way to prevent problems with corky ringspot is to avoid introducing TRV onto your property. Be sure to grow potatoes from seed that is certified as being free of TRV. Currently, seed potatoes produced in Wisconsin are considered TRV-free. Also avoid introducing the virus on infected ornamental plants. Carefully inspect ornamentals (see above for a partial list of susceptible species) prior to purchase for symptoms caused by TRV and DO NOT buy symptomatic plants. Alternatively (and preferably), avoid growing susceptible species altogether, and grow plants that are not susceptible to TRV. Non-susceptible plants include, but are not limited to, annual phlox, carnation, carrot, devil’s trumpet (downy thorn-apple), Scotch spearmint, sorrel, sweet William, zinnia and zombie cucumber.

To limit potential spread of TRV, routinely decontaminate tools (e.g., knifes or other cutting tools) that come into contact with potentially infected plant material (e.g., whole tubers that are cut into seed pieces, or ornamentals that are being divided). Also decontaminate tires, tools (e.g., spades or hoes) and any other object (e.g., shoes or boots) that might transport stubby-root nematode-infested (and thus TRV-infested) soil from field to field. First rinse any excess plant tissue or soil from these items, then treat them for at least 30 seconds in a solution that is a combination of 1% sodium lauryl sulfate and 1% Alconox (an industrial detergent). Trisodium phosphate (available at most local hardware stores) can also be used.

Also consider routinely testing soils for the presence of stubby root nematodes. Knowing the level of these nematodes in a field can provide information on the likelihood that TRV will spread should the virus be introduced.

Finally, DO NOT ever produce seed potatoes in fields with a history of corky ringspot or other TRV diseases. Also avoid using infested fields for potato or other vegetable production. If you decide to use a TRV-infested field for non-seed potato production, be sure to grow a TRV-resistant potato variety. The potato varieties ‘Castile’, ‘Millennium Russet’, ‘Red Pearl’, ‘Symfonia’, and ‘St. Johns’ have all been reported to have at least moderate levels of resistance to TRV.

For more information on corky ringspot:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Tom German, Amanda Gevens and Russell Groves for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Field and Forage, Disease - Vegetable

Common Corn Smut

July 14, 2015 hudelson

UW Plant Disease Facts

Authors: Tianna Jordan*, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0031

Common corn smut leads to tumor-like galls on corn filled with a sooty, black powder.

What is common corn smut?

Common corn smut is a fungal disease that affects field, pop, and sweet corn, as well as the corn relative teosinte (Zea mexicana). Common corn smut is generally not economically significant except in sweet corn where relatively low levels of disease make the crop aesthetically unappealing for fresh market sale and difficult to process for freezing or canning. Interestingly, the early stages of common corn smut are eaten as a delicacy in Mexico where the disease is referred to as huitlacoche (see UW Plant Disease Facts D0065, Huitlacoche).

What does common corn smut look like?

Common corn smut leads to tumor-like swellings (i.e., galls) on corn ears, kernels, tassels, husks, leaves, stalks, buds and, less frequently, on aerial roots. Some galls (particularly those on leaves) are small and hard. More typically, however, galls are fleshy and smooth, silvery-white to green, and can be four to five inches in diameter. As fleshy galls mature, their outer surfaces become papery and brittle, and their inner tissues become powdery and black. Galls eventually rupture, releasing the powder (i.e., the spores of the causal fungus).

Where does common corn smut come from?

Common corn smut is caused by the fungus Ustilago maydis, which can survive for several years as spores in soil and corn residue. Spores are spread by wind or through water splashing up onto young plants. Spores can also be spread through the manure of animals that have eaten infected corn. U. maydis most typically infects corn ears via the silks. The fungus can also enter plants through wounds caused by insect feeding, hail, or injuries from machinery. Infection is favored by warm weather (79-93°F) and moderate rainfall. Corn grown in low fertility soils or soils with excessive nitrogen also has a greater likelihood of infection.

How do I save plants with common corn smut?

Once galls have formed, treatment is not possible. Remove and burn (where allowed by local ordinance) or bury smut galls before they burst to prevent spores from spreading and overwintering. In order for eradication to be effective, you will need to coordinate efforts with your neighbors. Fungicides are currently not an effective control against common smut.

How do I avoid problems with common corn smut in the future?

Plant resistant corn varieties whenever possible. Check with your favorite sweet corn seed supplier for available varieties. Also, avoid injuring plants (e.g., when weeding) to reduce possible entry points for the smut fungus. Maintain well-balanced soil fertility (specifically nitrogen) based on a soil nutrient test. Use crop rotation (see University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden for details) to allow time for corn smut spores to naturally die off in the soil.

For more information on common corn smut:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*Completed as partial fulfillment of the requirements for Plant Pathology 558 at the University of Wisconsin Madison.

Thanks to Allyse Engle, Nancy Engle, Taylor Jordan, Nicholas O’Connor, Chris Parise, Karen Schleis and Trisha Wagner for reviewing this document .

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Field and Forage

Charcoal Rot of Soybean

July 14, 2015 hudelson

UW Plant Disease Facts

Pest Alert

Authors: Carol Groves and Damon Smith, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0029

What is charcoal rot?

Charcoal rot, also known as summer wilt or dry weather wilt, is a fungal disease of soybean that most commonly occurs in plants that are under heat and water stress. Charcoal rot is most prevalent in the southern United States, but can occur in the North Central region when weather is hot and dry. In Wisconsin, charcoal rot is observed most often in fields with sandy soils.

A dusty, gray discoloration of stems and roots is characteristic of charcoal rot of soybean. (photo courtesy of Theresa Hughes)

What does charcoal rot look like?

Plants suffering from charcoal rot may display premature yellowing of their top leaves, as well as premature leaf drop that may be mistaken for normal plant maturity. Plants with charcoal rot often initially wilt in the midday heat and then recover at night. Eventually permanent wilting will occur. In some cases, the upper third of a plant may have unfilled, flat seedpods. At flowering, a light gray discoloration develops in the surface tissues of both tap and secondary roots, as well as lowers stems. These tissues will appear as if they have been dipped in charcoal dust, hence the name of the disease. The dusty appearance is due to the presence of tiny survival structures (called microsclerotia) of the fungus that causes the disease.

Where does charcoal rot come from?

Charcoal rot is caused by the fungus Macrophomina phaseolina which has an extensive distribution and is known to infect over 500 plant species including corn (where it causes charcoal stalk rot), alfalfa, and many ornamental and weed species. M. phaseolina can survive for two or more years in dry soils as microsclerotia embedded in plant residue. In wet soils however, microsclerotia do not survive for more than seven to eight weeks. Hyphae (i.e., fungal threads) of the fungus typically do not survive in soil for more than seven days. Infections of M. phaseolina primarily occur in the spring when soil moisture is high. The fungus enters plants via roots and then grows very slowly until plants reach their reproductive stage (usually coinciding with the hottest, driest part of the growing season). Then more extensive colonization of plant tissue occurs. M. phaseolina is most active when soil temperatures are high (80 to 95°F), unlike many other soilborne, disease-causing fungi, which have reduced activity when soil temperatures are high.

How can I save a soybean crop with charcoal rot?

By the time that typical symptoms of charcoal rot are evident, control of charcoal rot is difficult, and losses in yield are likely inevitable. Foliar fungicides and fungicide seed treatments have no effect on charcoal rot.

How can I avoid problems with charcoal rot in the future?

Plant high quality, pathogen-free seed to prevent introduction of the charcoal rot pathogen into fields that are not currently infested. In fields where M. phaseolina is already present, any cultural practices that minimize plant stress will reduce the risk of charcoal rot. Use tillage practices (e.g., no-till) that maintain soil moisture, and irrigate where possible during dry periods to reduce drought stress. Lower plant populations and maintain good weed control to minimize stress from competition for soil nutrients. In addition, optimize soil fertility levels, particularly phosphorus. Rotations with wheat may provide some control of charcoal rot. However, because M. phaseolina has a wide host range (including corn), crop rotation may not provide sufficient control of charcoal rot. A moderate level of partial resistance is known in soybean varieties in maturity groups IV and higher. Unfortunately these varieties are not suitable for production in Wisconsin. Whether partial resistance is present in commercial varieties suitable to be grown in Wisconsin (maturity groups I and II) is not known.

For more information on charcoal rot of soybean:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Craig Grau, Kevan Klingberg and Ken Williams for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional agriculture-related questions at https://extension.wisc.edu/agriculture/ask-an-agriculture-question/.

Disease - Field and Forage

Brown Stem Rot of Soybean

July 6, 2015 hudelson

UW Plant Disease Facts

Authors: Carol Groves and Damon Smith, UW-Madison Plant Pathology

Last Revised: 02/28/2024

D-number: D0026

What is brown stem rot?

Brown stem rot (BSR) is a disease of soybean that was first observed in central Illinois in 1944 and is now prevalent throughout the North Central States of the US, and Canada. BSR has been identified as the third most important disease of soybeans in Wisconsin, attributable to the expansion of soybean acreage and shorter crop rotations used in the state. The agronomic impact of BSR is greatest in high yield potential environments. BSR negates the benefits of management practices intended to increase yield potential.

Brown pith discoloration of soybeans suffering from brown stem rot (top) compared with the white pith of a healthy soybean plant (bottom). (photo courtesy of Craig Grau)

What does brown stem rot look like?

Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effects of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development).

Foliar symptoms of BSR can be confused with those of sudden death syndrome (see UW Plant Disease Facts D0107, Sudden Death Syndrome of Soybean). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

Where does brown stem rot come from?

BSR is caused by the soilborne fungus Phialophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss.

P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no-till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, P. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

Interveinal chlorosis and necrosis typical of brown stem rot. (photo courtesy of Craig Grau)

How can I save a soybean crop with brown stem rot?

BSR cannot be controlled once plants have been infected. Foliar fungicides and fungicide seed treatments have no effect on the disease.

How can I avoid problems with brown stem rot in the future?

Use crop rotations of two to three years away from soybean with a non-host crop (e.g., small grains, corn, or vegetable crops), as well as tillage methods that incorporate plant residue into the soil. Both of these techniques will help reduce BSR pathogen populations by promoting decomposition of soybean residue. Also, make sure that soil fertility and pH are optimized for soybean production to avoid overly low phosphorus and potassium levels, as well as overly low soil pH. Finally, grow soybean varieties with resistance to BSR. Complete resistance to BSR is not available in commercial varieties. However several sources of partial resistance that provide moderate to excellent BSR control are available. Also, some, but not all, varieties of soybean cyst nematode (SCN) resistant soybeans also are resistant to BSR. Most soybean varieties with SCN resistance derived from PI 88788 express resistance to BSR. However, the same is not true of varieties with SCN resistance derived from Peking. Therefore growers should consult seed company representatives about BSR resistance when selecting a variety with SCN resistance derived from this source.

For more information on brown stem rot of soybean:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Craig Grau, Bryan Jensen and Ken Williams for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.

Submit additional agriculture-related questions at https://extension.wisc.edu/agriculture/ask-an-agriculture-question/.